(1) Field of the Invention
The present invention is concerned with an imaging device provided with means for applying bias-light, suitable for use as an imaging device having a color selective stripe filter. More specifically, the present invention relates to an imaging device having improved lag characteristics due to an improved manner of application of a bias-light which is a faint light adapted to be evenly applied to an image pick up portion for attenuating a lag.
The present invention can be suitably applied particularly, but not exclusively, for photoconductive image pick up tubes.
(2) Description of the Prior Arts
In picking up the image of a low light level object by an image pick up tube, there is a practical problem of lag. Conventionally, in order to avoid this lag in photoconductive image pick up tube, it has been proposed to uniformly apply a faint light onto the photo-conductive layer of the image pick up tube. This method is referred to as "bias-light method."
The lag in the photoconductive image pick up tube can be largely devided into two components of photoconductive lag and capacitive lag. The photoconductive lag depends on the life span of carriers generated in the photoconductive body by the application of the light until the extinction thereof due to a recombination in the photoconductive body, or on the running time in which the carriers drift through the photoconductive body, and is largely affected by carrier traps existing the photoconductive body. On the other hand, the capacitive lag is determined by a time constant .tau. = RC which is a product of the capacitance C of a target consisting of a photoconductor corresponding to a dielectric body and of a transparent conductive electrode scanned by an electron beam, and a resistance R by which the electron beam is encountered.
In general, two advantages are expected from an application of a faint light on the photoconductive layer on the photoconductive element of the tube.
One of these advantages is that the effective life span of the carrier or the running time thereof is conveniently reduced due to a reduction of the effect of the traps, while the other being a reduction of differential resistance of the scanning electron beam due to a displacement of the zero operation point of the target.
These effects are both effective to reduce the lag on the photoconductive tube.
This advantageous phenomenon constitutes the principle of the bias-light method for reducing the lag, and has been an essential factor in designing a camera.
Meanwhile, TV camera systems have been proposed which have a color selective stripe filter on the surface of the image pick up tube, for performing a sampling for respective colors, so as to reduce the number of image pick up tubes, through so called multiplexing of the colored picture in the spatial frequency domain.
FIGS. 1a and 1b in combination show an example of a surface of an image pick up tube having a color stripe filter, for use in TV camera system, in which FIG. 1a shows a plan, while FIG. 1b shows a cross-section of the surface.
Referring to those Figures, the color selective stripe filter 1 includes, for example, stripe filters 1R, 1G and 1B, capable of selecting colors of red, green and blue, respectively. The color selective filter 1 is adhered to a substrate 2 consisting of a glass plate, and is adapted to perform a spatial sampling of color lights 6 from an object. In general, a transparent surface layer or film 3 is provided for protecting the filter and for rendering the surface of the filter smooth, since the surface of the filter 1 usually involves a height differential depending on the colors. A conductive film 4 is provided on the transparent film 3, for playing a role of a signal electrode for imparting a bias potential.
The conductive film 4 is divided into two or three sections, or alternatively, not divided, depending on the methods of electrically demodulating the spatially multiplexed light signal, in relation to the direction of the color selective stripe filter or the pitch thereof.
The conductive film 4 is covered with a photoconductive layer 5 for converting the light into charges for a scanning by an electron beam, so as to produce an electrical signal at the signal electrode.
In most of the image pick up tubes provided with the color selective stripe filter of this kind, the photoconductive body is constituted by antimony trisulfide the production of which has been commercially settled. The reduction of the lag in the photoconductive body of this material by the application of the bias light is so small that the bias-light method has seldom been applied for the image pick up tube.
However, the bias-light method is becoming popular, in correspondence with the development of photoconductive materials such as lead oxide and cadmium selenide which exhibit a remarkable effect of reduction of lag by the application of the bias light, particularly in color TV camera relying upon multi-tube system e.g. 3-tube system.
The use of the bias light is, however, still not popular, for the system incorporating a color selective stripe filter, partly because of an increase of dark current and shortening of the life span which are attributable to an irregularity of contact between the photoconductive body of these materials and the signal electrode, and partly because of problems during processing such as an undesirable change of color of the color selective stripe filter due to a high-temperature treatment during the manufacture of the photoconductive layer.
Under these circumstances, the demand for a compact, light-weight and easily adjustable TV camera system is increasing, which in turn activates the development of image pick up tubes having a color selective stripe filter, as well as the study and investigation on the photoconductive layer itself. Consequently, photoconductive image pick up tubes having improved sensitivity and durability have become available.
The present inventors have attempted to apply the technique of the bias-light method which has been popular for the multi-tube system, e.g. 3-tube system, to the photoconductive image pick up tube having a color selective stripe filter.
Typical bias light method technique incorporates an annular light source provided just in front of the surface of an image pick up tube, as shown in FIGS. 2a and 2b, which is detailed in Japanese Patent Publication No. 24906/1970 and so on. Alternatively, it has been proposed in Japanese Patent Publication No. 13973/1976 to use a clad rod arranged optically opposing to the light-receiving surface of the image pick up tube, so as to make use of the light radiated externally of the clad rod.
These proposed techniques are intended for imparting to the photoconductive layer a bias light of small shading, by applying light uniformly to the face plate or the light-receiving surface of the image pick up tube.
More specifically, referring to FIGS. 2a and 2b, a ring-shaped box 8 is provided on the glass substrate plate 2 of the image pick up tube, so as to encircle a light source 9 which may be an incandescent lamp or a light emitting diode. The inner peripheral surface of the ring-shaped box 8 constitutes a light diffusion surface 20, while the inner surface 21 of the outer periphery of the box 8 and the inner surface 22 of the cover of the box in combination act as reflective diffusion surfaces. The surface 23 in contact with the glass substrate plate 2 of the image pick up tube is made non-transmissive. Thus, the face plate of the image pick up tube is adapted to receive the light diffused from all directions.
A test was conducted to apply the bias light onto an image pick up tube having a color selective stripe filter by means of the illustrated light source. The test showed, however, that the shading is impractically large, although the lag is reduced to some extent. The shading was found to be large in the direction perpendicular to the longitudinal direction of the color selective stripe filter, i.e. in the direction in which the color selective stripe filter is arrayed, and to have a pattern biased for respective colors. These phenomenon have been proved to be inherent in the image pick up tube having a color selective stripe filter.